In the past few years, lentiviral gene transfer vectors have gained a considerable interest among the gene therapy community. This unmatched reputation is due to their ability to efficiently and stably transfer therapeutic or reporter genes to a large variety of cells and tissues of key importance for therapeutic intervention, such as hematopoietic stem cells, brain, liver and retina (see [1-4], among others). Lentiviral vectors achieve high transduction efficiency irrespective of the proliferative status of the target cells, thus circumventing one of the main limitations of oncovirus-derived retroviral vectors in which transduction is restricted to dividing cells.
This advantage is reflected in a number of successful preclinical tests in various animal models of human diseases (see [5-11], among others), and will undoubtedly translate into their exploitation in a growing number of clinical trials.
More recently, lentiviral vectors have also been proven as promising vaccination vectors. Most studies to date have focused on the induction of cellular immune responses in the field of anti-tumoral immunotherapy [12-18], a few have also focused on the induction of protective cellular immunity against viruses [19-21].
Their capacity to transduce non-dividing dendritic cells (DCs) with high efficiency, ex-vivo [12, 14, 15, 19] as well as in vivo [13, 18, 22], accounts for their ability to elicit strong CTL responses.
Indeed, reports using lentiviral vectors to stimulate anti-tumor immunity show very promising results. Human DCs transduced by lentiviral vectors expressing tumor antigens stimulate specific CTL responses in vitro [12, 14, 15, 18]. In mice, the injection of lentiviral vector particles or lentiviral vector transduced DCs induce strong and specific anti-tumor cellular immune responses [13, 15, 18] and confer protection from tumor challenge [12, 17]. These responses are more potent than the ones obtained by classical immunization with peptide plus adjuvant [13] or even peptide-pulsed antigen presenting cells (APCs) whether assayed ex vivo [20] or in vivo [16, 21]. The observed advantage could be due to the continuous presentation of the antigen in vector-transduced APCs in contrast to the transient antigen presentation that ensues from peptide pulsing of APCs [16, 20].
To date, little is known of the ability of lentiviral vectors to stimulate an antibody-based protective immunity. Nevertheless, their high transduction efficiency could also grant them a strong ability to induce humoral immunity. Moreover, their seemingly preferential tropism for DCs when injected in vivo [13, 18, 22] could further enhance their capacity to elicit a protective antibody-based immunity, since DCs are powerful stimulators of CD4 T cells, which are needed for the correct development of a B-cell based immune response. Furthermore, in a vaccination scenario, the stable expression of the antigen from transduced cells might preclude the need for several injections.
Indeed, it is widely accepted that the humoral immune response is the essential component of protective immunity against West Nile Virus (WNV) [23-25]. The envelope E-glycoprotein from WNV, which possesses neutralizing epitopes [26], elicits protective immune responses when injected as a recombinant antigen [27] or expressed either by naked DNA [28] or a replicative measles vector [29]. The fact that passive transfer of neutralizing antibodies to the soluble form of the envelope E glycoprotein (sE) from WNV strain IS-98-ST1 protected mice from WNV encephalitis further demonstrates that the humoral response is sufficient for protection against WNV challenge [29].
WNV is a mosquito-borne flavivirus in the Japanese encephalitis serocomplex of the Flaviviridae family. It is transmitted in natural cycles between birds and mosquitoes, but it can infect many species of mammals [30]. Zoonotic WNV recently became a major health concern in North America, the Middle East, and Europe, due to the emergence of a more virulent strain in Israel in 1998 and then in New York in 1999 (Isr98/NY99). Severe clinical manifestations of WNV infection essentially involve the central nervous system and can cause significant mortality in humans and in a large range of animal species, particularly horses and birds (for review see [30]). As a consequence, an urgent demand exists for the development of an efficient vaccine that can provide quick, strong and long lasting immunity.